Agilent Measurement Journal - Issue 5 - 2008 - (Page 24)

combined in a Multicast/Broadcast over Single Frequency Network (MBSFN) with protection from delay spread of up to 10 km. This very long CP means there are only three symbols per timeslot, but this capacity loss is counteracted by the doubling up of the subcarriers. Table 5. Cyclic preﬁx conﬁgurations for downlink FS1 signal generation, and power boosting are needed to allow the signal to be measured. Many of these parameters can be estimated or extracted from the signal under test, but for the remainder of the testing they must be entered manually. Figure 11 shows one of the many signal conﬁguration menus of the Agilent 89601A vector signal analysis software which allows users to specify numerous parameters for measuring the LTE downlink. Using this software, many key parametric and demodulation measurements can be performed to determine the downlink performance of LTE devices. Figure 12 shows six different views of an LTE downlink signal. The top middle trace shows the auto-detected signals (P-SCH, S-SCH, PBCH, PCFICH, PDCCH, and RS) and one shared channel — PDSCH3 at 64QAM. Each channel type is colorcoded to enable the different elements of the downlink to be identiﬁed on the other traces and independently measured. As well as auto-detecting the channels, this trace also measures the channel-speciﬁc EVM, the channel power and the modulation type. The top right trace gives further information about the modulation quality including the frequency error and the IQ offset and gain imbalance. The top left trace shows the IQ constellation of the signal, which shows everything from the Zadoff-Chu RS through the QPSK control channels to the 64QAM of the PDSCH3. The bottom left trace is a traditional power-versus-frequency spectrum, which shows this is a 5-MHz allocation. This is conﬁrmed in the bottom middle trace which shows the EVM by subcarrier. Note that the X scale shows a range of 300 subcarriers which is an occupied bandwidth of 4.5 MHz at 15 kHz per subcarrier. EVM per subcarrier is an important measure since the EVM is likely to degrade at the channel edges due to the transmit ﬁlter. The ﬁnal trace at the bottom right shows how the EVM varies in time across the 10-ms capture interval. This can be useful in determining downlink performance after transient events such as a power change. Layers Antenna ports CP in Ts by symbol number 0 1 2 34 Normal Df=15 kHz 160 144 144 144 144 Df=15 kHz 512 512 512 512 512 Extended Df=7.5 kHz 1024 1024 1024 – – 5 144 512 – 6 144 – – Verifying the downlink Verifying the early functionality and performance of the LTE downlink requires ﬂexible signal analysis. Figure 10 shows an outline of the downlink signal-generation process. First the incoming bit stream is scrambled using pseudo-random sequence generation. Next, the bits are mapped to a modulation symbol format. For example, the PDSCH can be mapped to QPSK, 16QAM or 64QAM containing two bits, four bits and six bits per symbol, respectively. Layer mapping is then applied to support various antenna conﬁgurations and precoding can be applied to adjust the phase and amplitude of each layer for each antenna. Next, resource mapping is applied and a downlink OFDMA signal is generated for each antenna. When the signal is received, it is demodulated using the inverse of this process. In the real world, the base station and UE communicate with each other via signaling, exchanging essential information about the uplink and downlink signal composition. However, in the early phases of testing, this information exchange does not take place. Nevertheless, parameters for such things as resource mapping, modulation scheme, sequence generation, OFDM Code words Scrambling Modulation mapper Layer mapper Precoding Resource element mapper OFDM signal generation Scrambling Modulation mapper Resource element mapper OFDM signal generation Figure 10. Downlink signal generation 24 Agilent Measurement Journal

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 5 - 2008

Agilent Measurement Journal Issue 5 Finding a New Path when Assumptions Break Down Contents Testing MIPI D-PHY Protocols Oscilloscope Firmware Update DNA Replication Joining the LTE/SAE Trial Initiative Testing Proteins Praise for DC Power Analyzer Scripting Features for AMDS Testing Hybrid PCBA Systems Improving the Gene Expression System Workflow Turning a “Good Enough” Test Strategy into One That’s Reliable, Repeatable and Traceable Understanding the Effects of Limited-Bandwidth Channels on Digital Data Signals Introducing the 3GPP LTE Downlink Validating the 26 Validating the Physical and Protocol Layers in DDR Memory Interfaces Understanding Total Jitter Measurements of Low Probabilities Using Behavioral-Model Simulation to Accurately Predict First-Order PLL Performance Creating Synchronous High-Frequency Sampling across Multiple Digitizers Overcoming the Challenges of Testing FlexRay Networks Understanding Operating Life and Repeatability of Electromechanical Switches and their Effect on Total Cost of Ownership Implementing Micro and Nano LC/MS Techniques for High Sensitivity Lipidomics Analysis

Agilent Measurement Journal - Issue 5 - 2008

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